A turboshaft engine is a gas turbine engine comprised, in flow series arrangement, of a gas generator which extracts energy from fuel and produces pressurized hot gas, and a free turbine. The free turbine extracts a portion of the energy of the hot gas and converts this energy into mechanical power in the form of a rotating shaft. Because the free turbine is not mechanically coupled to the the gas generator its shaft is free to rotate at slower speeds, and therefore can be used to drive a propeller. The gas generator includes a compressor, a combustor, and a turbine coupled to the compressor via a shaft.
Turboshaft engines have been successfully used as power plants for driving the rotor blades of helicopters. Sometimes, helicopters are required to fly various low altitude missions that demand large, rapid changes in power from their turboshaft engine. One such mission is terrain avoidance when flying very close to the ground. Another mission is called masking/unmasking. In this mission the helicopter is initially hovering hidden behind an object, then it requires a rapid increase in power to quickly rise above the object to reconnoiter or launch a weapon followed by an equally rapid decrease in power to quickly descend to safety behind the object. Helicopters, like fixed wing aircraft, also fly missions necessitating steep dives and climbs. Consequently, there has developed a need, especially in military helicopters, for a turboshaft engine that can rapidly change its power output.
One approach to rapidly change the power output of a turboshaft engine is to move the engine's throttle. For example, as the throttle is pushed forward, to its maximum position, the fuel flow to the engine increases resulting in increasing power output. The speed or rate at which the power increases is related to the speed or rate at which the fuel flow increases. However, the speed or rate at which the fuel flow increases is limited by a number of factors including combustor delay, overheating of the turbine blades, compressor stall, and lag due to mass inertia in accelerating the engine's rotating components.
Another approach to this problem is to incorporate rotatable vanes at the inlet of either the compressor or the turbine. The disadvantages of these vanes are that they add considerable weight to the aircraft and require complicated actuation devices.
Accordingly, a need exists for a turboshaft engine that can rapidly change its power output without any of the previously mentioned disadvantages or limitations.